1. Field of the Invention
The present invention relates to a method of forming a photo mask, and more particularly, to a method of forming a multi-layer photo mask.
2. Description of the Prior Art
In semiconductor processing, a designed pattern is initially formed on a photo mask and then the pattern of the photo mask is transferred onto the surface of a semiconductor wafer by a photolithography process so as to define the pattern of integrated circuits. The photo mask with poor quality is not in favor of the pattern transfer which may result in the poor electrical performance of semiconductor products and the high cost of processing. Therefore, how to form a photo mask with good quality becomes a very important issue.
Please refer to FIG. 1 to FIG. 4. FIG. 1 to FIG. 4 are schematic diagrams of a method of forming a phase-shift photo mask 26 according to the prior art. A method of forming a phase-shift photo mask is performed on a photo mask substrate 10 made of quartz. The photo mask substrate 10 is defined by a plurality of predetermined regions 20 according to a designed pattern required by a semiconductor process. During the method of forming the phase-shift photo mask 26, an anti-reflective layer 12, a phase shifter 14, a first opaque layer 16 made of chromium (Cr), and a first photoresist layer 18 are formed in sequence, as shown in FIG. 1. The anti-reflective layer 12 is used for enhancing the light transmission rate of the photo mask substrate 10. The phase-shift layer 14 is used for driving the transmitting light to generate a phase-shift angle for about 180.degree..
Next, an exposure process is performed by using laser beam or electronical beam (E-beam) to expose the first photoresist layer 18. Then a development process is performed to form a second photoresist layer 19 on the predetermined region 20 of the photo mask substrate 10, as shown in FIG. 2. Afterward, an etching process is performed to vertically remove the first opaque layer 16 outside the predetermined region 20 so as to form a second opaque layer 17 where the designed pattern is defined, as shown in FIG. 3. Finally, a resist stripping process is performed to completely remove the second photoresist layer 19 so that the phase-shift photo mask 26 is completed, as shown in FIG. 4
According to the prior art method of forming the phase-shift photo mask 26, the designed pattern is defined on the second opaque layer 17 wherein a line width W and a line space S form a minimum pitch 25. The minimum line width and the minimum line space of the pattern on the phase-shift photo mask 26 are both limited, because of a certain resolution of laser beam or E-beam. Therefore, there will be a limitation in the minimum pitch 25 of the phase-shift photo mask 26. Since the line width of the pattern is related to the pitch 25 of the phase-shift photo mask 26, the phase-shift photo mask 26 with the minimum pitch 25 may not be applied to a semiconductor process with a narrower width. Although the minimum pitch 25 could be further reduced by changing the light source used in the exposure process and the material of the first photoresist layer 18, this will greatly increase the process cost and hence not meet the economic efficiency.